cmake : make LLAMA_NATIVE flag actually use the instructions supported by the processor (#3273)

* fix LLAMA_NATIVE

* syntax

* alternate implementation

* my eyes must be getting bad...

* set cmake LLAMA_NATIVE=ON by default

* march=native doesn't work for ios/tvos, so disable for those targets. also see what happens if we use it on msvc

* revert 8283237 and only allow LLAMA_NATIVE on x86 like the Makefile

* remove -DLLAMA_MPI=ON

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Co-authored-by: netrunnereve <netrunnereve@users.noreply.github.com>